The present invention relates to a method for determining a control signal or drive voltage for a piezoelectric actuator of an injection valve.
An injection valve for injecting fuel into the combustion chamber of an internal combustion engine having a high pressure system (common rail system) is discussed in German Published Patent Application No. 197 328 02, which corresponds to U.S. Pat. No. 6,021,760. This injection valve has two valve seats against which a valve closing element is moved when activated by a piezo actuator. If the valve closing element starts out in a closing position at the first valve seat, it can be brought into an intermediate position between the valve seats and then into a second closing position at the second valve seat, with the aid of the piezo actuator. To accomplish this, the piezoelectric actuator is loaded to a control voltage which is a function of the pressure in the common rail system. On account of the voltage applied, the actuator stretches in the longitudinal direction and thereby moves the closing element in the direction of the second valve seat. To reverse the movement of the valve closing element in the direction of the first valve seat, the actuator is unloaded again.
By way of the sequence of movements of the valve closing element from one valve seat to the other, a short-term unloading of a valve control chamber, which is under high pressure, may be achieved, via whose pressure level the activating of a valve needle into an opening or closing position is performed. If the valve closing element is in an intermediate position between the two valve seats, fuel injection takes place. In this way, one can also produce a dual fuel injection, such as a pre-injection and a main injection.
The control of the valve member does not take place directly, but by a hydraulic transmission to a hydraulic coupler. When the piezoelectric actuator is loaded so strongly with voltage that the valve closing member moves from its valve seat, part of the fuel quantity present in the hydraulic chamber is squeezed out through its leakage passage. It is believed that this effect may be particularly large when the control valve is held at the second valve seat facing the high pressure area, since in this case the counteracting force may be particularly great because of the rail pressure. Recharging the low pressure area in the chamber of the hydraulic coupler takes place by a system pressure which, for example can be 15 bar, in practice. The recharging likewise is done via the leakage passage, but only at such time as the piezoelectric actuator is not activated.
In the case of the injection valve discussed above, however, the problem may arise, that the hydraulic coupler, as a rule, may not be completely recharged. The valve lift set at equal control voltages of the piezoelectric actuator can, therefore, be quite different, depending on the degree of recharging. The closer two injections follow one another, the less is the recharging of the coupler. It is also believed that it may be unfavorable that the amount of leakage becomes greater with a long trigger time of the actuator and with a longer loading period of the hydraulic coupler. In this case too, the recharging may not always be guaranteed, and so, a different valve lift is possible at an unchanged control voltage. Again, the different valve lift may have the subsequent disadvantage that the dosing of the injection quantity is imprecise, and, under certain circumstances, can have the effect that the desired injection of fuel does not take place if, because of the low recharging of the coupler, the valve is not positioned correctly, and, therefore, the nozzle needle is not opened.
An exemplary method according to the present invention, for determining the control voltage for a piezoelectric actuator of an injection valve, is believed to have the advantage that an optimal control voltage for the actuator may always be supplied, independently of the duration of the prior injection or its activation. It is also believed to be especially advantageous that, with the aid of the measured parameter, the injection valve may be positioned so that the requisite injection quantity is actually ejected, independently of the momentary filling level of the hydraulic coupler or the pressure prevailing in it. This may be particularly necessary with small dosings.
It is also believed to be especially advantageous that the pressure in the hydraulic coupler acts on the piezoelectric actuator and induces a voltage in it which is measurable at the output terminals. Because of this, advantageously, the pressure in the coupler, which acts on the actuator and induces a voltage in it, may be indirectly measured without a further sensor.
Furthermore, it is also believed to be advantageous that the pressure between two injections may be measured, for instance, shortly before the beginning of the next injection. That should at least better guarantee that the pressure present at the moment in the coupler is measured.
In another exemplary method, the algorithm may be stored in the form of a table, so that there is simple access to the corresponding correlation values between the pressure and the control voltage.
If, however, the induced voltage lies below a predefined threshold, one may assume that no injection or no correct one will take place, because the coupler was not sufficiently filled. It is believed that this effect can be advantageously used for recognizing intermittent operation or recognizing a fault in the charging of the coupler.
It is also believed to be advantageous to adjust the control voltage proportionally to the pressure of the coupler. This adjustment can be determined with a factor by which, for example, the control voltage is multiplied. In particular, in the measurement of the pressure of the coupler shortly before the subsequent injection, it is believed to be advantageously at least better guaranteed that the actual degree of recharging of the coupler is considered.
The determination and the production of the control voltage for the actuator by a software program represents a simple solution, which also makes simpler the application to different engine types, since no mechanical changes have to be made.
It is also believed that an advantage may be provided by using the exemplary method for fuel injection for an internal combustion engine, especially since the calculation of the control voltage can be set individually for each cylinder of the engine.